1. Field of the Invention
This invention pertains to sample and hold circuits and more particularly to a sample and hold circuit having a nonlinear circuit, together with the sample capacitor, in a feedback loop to provide an extended range of input voltage amplitude capability.
2. Description of the Prior Art
When a sample is taken of a varying signal, a sample and hold circuit is used to capture a sample of the signal and to hold it for measuring or other purposes. The widest use of the sample and hold circuit is with high speed approximation analog to digital converters as part of floating point amplifiers.
The prior art sample and hold circuits used in these applications generally have a differential amplifier input stage having 100% inverse feedback, a sample transistor switch connected to a sample capacitor and to an output differential amplifier having 100% inverse feedback. The transistor switch is closed for a period of time allowing the input analog signal, applied to the input amplifier to charge the sample capacitor. When the transistor sample switch is opened, the sample is held with the output of the output amplifier representing the sample of the input analog signal. The output differential amplifier provides a very high input impedance to prevent discharging the sample capacitor. Any current required by the output amplifier must be extremely small because it will gradually change the charge on the capacitor. Leakage resistance in the component layout, internal leakage resistance of the capacitor, presence of moisture, etc. are factors that limit the length of time that the capacitor can accurately hold the charge. The sample capacitor is not in a feedback loop. The "on" resistance of the switch and any offset error in the output amplifier will cause an error in the output.
An improved prior art sample and hold circuit has the output of the output amplifier connected to the inverting input of the input amplifier thereby placing the sample capacitor in a feedback loop during the sampling time when the sample switch is turned on. This circuit provides inverse feedback around the entire circuit. The output therefore follows the input when the sample switch is closed regardless of errors at the switch, at the capacitor, or at the output amplifier. However, offset in the input amplifier is not corrected by the loop.
Furthermore, the transistor sample switch has an internal value of capacitance between the switch control line and the switch terminals. When the control line makes the step change of voltage to turn the transistor switch off, a current from the capacitance formed between the control terminal and the output terminal flows into the sample capacitor (pumped charge) changing its charge and causing an error. This capacitance will change only if the terminal voltage changes.
The sample and hold circuit of this invention places the sample capacitor in the feedback loop as part of an integrating circuit with a nonlinear circuit at the output of the integrater, the overall combination providing a very wide range of input analog signal capability. Furthermore, minimal circuitry is added to compensate for any pumped charge error.